Interesting, that "selection" by itself implies some higher
authority which defines what is good and bad (criterion for selection,
someone or something which desides say short or long fins of carp to kill off).
By saying "natural" you did not remove the need of such authority,
you just made it undefined but still implied. So, what physical law
do you think defines direction of evolution?
Well, all of them. More specifically, the reason why
there seems to be a direction (towards greater complexity,
greater size, greater intelligenc, etc) is that for each
of those variables, there's a strict lower bound, zero,
and we started much closer to there than whatever upper
bound there might be. This isn't my idea - I read it
somewhere else - but it's pretty obvious from a
mathematical standpoint.
My answer is - it is the old good second law of thermodynamics,
which in fact defines existance of any process whatsoever.
Physical laws are generally descriptive, not prescriptive.
The second law of thermodynamics, in particular, isn't either.
It merely defines the notions of directionality of time and
entropy in relation to each other.
The requirement of second law of thermodynamics to Life
is "catalyze (accelerate) increase of entropy as much as you can".
There is no such requirement whatsoever. Life doesn't have to
do anything. Survival isn't a matter of increasing entropy.
And even if it were, it has nothing whatsoever to do with the
second law of thermodynamics.
In fact, you're saying over time, there will be more certainty
in exactly what kinds of organisms will survive, which appears
incorrect given the current level of biodiversity and implies
reduction in entropy (not strictly, but to be pedantic, nothing
you've ever said has anything to do with entropy).
Mechanism how this requirement is enforced is simple:
1) The catalysts which can not do this as fast as fundamentaly possible
are killed off - by advance of others which can do it faster and
pushing them from particular thermodynamical niche.
2) At the other hand, catalysts which are not able to use sufficent
part of releazed energy for own repair and propagation (regardless to
question how efficient they are as accelerators) just decompose
by themselfs, because any complex catalyst is not thermodynamically
stable.
In short, organisms that can use external resources to
reproduce rapidly will be more successful than those that
aren't as good at doing so. That has nothing to do with
thermodynamics and doesn't explain anything. Neither
complexity, nor faster metabolism contributes directly to
reproductive success.
This simple approach explains why Life increases its complexity
and even in what direction it evolves - in direction of higher energy
conversion (faster entropy increase) and higher ability to self-repair
and multiply.
Any change, be it on genetic level of single organism,
or on macro level (say, certain polytical system) which goes contrary to
this principle, is doomed to cause extintion.
There seem to be plenty of bacteria species around that
are nowhere near as complicated as various extinct species.
And that's not to mention viruses.
Dan.